Patient alignment system with external measurement and object coordination for radiation therapy system

ABSTRACT

A patient alignment system for a radiation therapy system. The alignment system includes multiple external measurement devices which obtain position measurements of components of the radiation therapy system which are movable and/or are subject to flex or other positional variations. The alignment system employs the external measurements to provide corrective positioning feedback to more precisely register the patient and align them with a radiation beam. The alignment system can be provided as an integral part of a radiation therapy system or can be added as an upgrade to existing radiation therapy systems.

RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application No.10/917,023, filed Aug. 12, 2004, which claims the benefit of the U.S.Provisional Application No. 60/494,699, filed Aug. 12, 2003 and U.S.Provisional Application No. 60/579,095 filed Jun. 10, 2004 both entitled“Precision Patient Alignment and Beam Therapy System”.

GOVERNMENT SUPPORT

This invention was made with United States Government support under theDAMD17-99-1-9477 and DAMD17-02-1-0205 grants awarded by the Departmentof Defense. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of radiation therapy systems. Oneembodiment includes an alignment system with an external measurementsystem and local feedback to improve accuracy of patient registrationand positioning and to compensate for misalignment caused by factorssuch as mechanical movement tolerances and non-strictly rigidstructures.

2. Description of the Related Art

Radiation therapy systems are known and used to provide treatment topatients suffering a wide variety of conditions. Radiation therapy istypically used to kill or inhibit the growth of undesired tissue, suchas cancerous tissue. A determined quantity of high-energyelectromagnetic radiation and/or high-energy particles is directed intothe undesired tissue with the goal of damaging the undesired tissuewhile reducing unintentional damage to desired or healthy tissue throughwhich the radiation passes on its path to the undesired tissue.

Proton therapy has emerged as a particularly efficacious treatment for avariety of conditions. In proton therapy, positively charged protonsubatomic particles are accelerated, collimated into a tightly focusedbeam, and directed towards a designated target region within thepatient. Protons exhibit less lateral dispersion upon impact withpatient tissue than electromagnetic radiation or low mass electroncharged particles and can thus be more precisely aimed and deliveredalong a beam axis. Also, upon impact with patient tissue, theaccelerated protons pass through the proximal tissue with relatively lowenergy transfer and then exhibit a characteristic Bragg peak wherein asignificant portion of the kinetic energy of the accelerated mass isdeposited within a relatively narrow penetration depth range within thepatient. This offers the significant advantage of reducing delivery ofenergy from the accelerated proton particles to healthy tissueinterposed between the target region and the delivery nozzle of a protontherapy machine as well as to “downrange” tissue lying beyond thedesignated target region. Depending on the indications for a particularpatient and their condition, delivery of the therapeutic proton beam maypreferably take place from a plurality of directions in multipletreatment fractions to achieve a total dose delivered to the targetregion while reducing collateral exposure of interposed desired/healthytissue.

Thus, a radiation therapy system, such as a proton beam therapy system,typically has provision for positioning and aligning a patient withrespect to a proton beam in multiple orientations. In order to determinea preferred aiming point for the proton beam within the patient, thetypical procedure has been to perform a computed tomography (CT) scan inan initial planning or prescription stage from which multiple digitallyreconstructed radiographs (DRRs) can be determined. The DRRssynthetically represent the three dimensional data representative of theinternal physiological structure of the patient obtained from the CTscan in two dimensional views considered from multiple orientations andthus can function as a target image of the tissue to be irradiated. Adesired target isocenter corresponding to the tissue to which therapy isto be provided is designated. The spatial location of the targetisocenter can be referenced with respect to physiological structure ofthe patient (monuments) as indicated in the target image.

Upon subsequent setup for delivery of the radiation therapy, aradiographic image is taken of the patient, such as a known x-ray image,and this radiographic image is compared or registered with the targetimage with respect to the designated target isocenter. The patient'sposition is adjusted to, as closely as possible or within a giventolerance, align the target isocenter in a desired pose with respect tothe radiation beam as indicated by the physician's prescription. Thedesired pose is frequently chosen as that of the initial planning orprescription scan.

In order to reduce misalignment of the radiation beam with respect tothe desired target isocenter to achieve the desired therapeutic benefitand reduce undesired irradiation of other tissue, it will be appreciatedthat accuracy of placement of the patient with respect to the beamnozzle is important to achieve these goals. In particular, the targetisocenter is to be positioned translationally to coincide with thedelivered beam axis as well as in the correct angular position to placethe patient in the desired pose in a rotational aspect. In particular,as the spatial location of the Bragg peak is dependent both upon theenergy of the delivered proton beam as well as the depth andconstitution of tissue through which the beam passes, it will beappreciated that a rotation of the patient about the target isocentereven though translationally aligned can present a varying depth andconstituency of tissue between the initial impact point and the targetisocenter located within the patient's body, thus varying thepenetration depth.

A further difficulty with registration and positioning is that aradiation therapy regimen typically is implemented via a plurality ofseparate treatment sessions administered over a period of time, such asdaily treatments administered over a several week period. Thus, thealignment of the patient and the target isocenter as well as positioningof the patient in the desired pose with respect to the beam is typicallyrepeatedly determined and executed multiple times over a period of daysor weeks.

There are several difficulties with accurately performing this patientpositioning with respect to the radiation treatment apparatus. Aspreviously mentioned, patient registration is performed by obtainingradiographic images of the patient at a current treatment session at theradiation therapy delivery site and comparing this obtained image withthe previously obtained DRR or target image which is used to indicatethe particular treatment prescription for the patient. As the patientwill have removed and repositioned themselves within the radiationtherapy apparatus, the exact position and pose of a patient will not beexactly repeated from treatment session to treatment session nor to theexact position and pose with which the target image was generated, e.g.,the orientation from which the original CT scan generated the DRRs.Thus, each treatment session/fraction typically involves preciselymatching a subsequently obtained radiographic image with an appropriatecorresponding DRR to facilitate the determination of a correctivetranslational and/or rotational vector to position the patient in thedesired location and pose.

In addition to the measurement and computational difficulties presentedby such an operation, is the desire for speed in execution as well asaccuracy. In particular, a radiation therapy apparatus is an expensivepiece of medical equipment to construct and maintain both because of thematerials and equipment needed in construction and the indication forrelatively highly trained personnel to operate and maintain theapparatus. In addition, radiation therapy, such as proton therapy, isincreasingly being found an effective treatment for a variety of patientconditions and thus it is desirable to increase patient throughput bothto expand the availability of this beneficial treatment to more patientsin need of the same as well as reducing the end costs to the patients orinsurance companies paying for the treatment and increase theprofitability for the therapy delivery providers. As the actual deliveryof the radiation dose, once the patient is properly positioned, is arelatively quick process, any additional latency in patient ingress andegress from the therapy apparatus, imaging, and patient positioning andregistration detracts from the overall patient throughput and thus theavailability, costs, and profitability of the system.

A further difficulty with accurately positioning the patient and thecorresponding target isocenter in the desired position and pose withrespect to the beam nozzle are the multiple and additive uncertaintiesin the exact position and relative angle of the various components of aradiation therapy system. For example, the beam nozzle can be fitted toa relatively rigid gantry structure to allow the beam nozzle to revolveabout a gantry center to facilitate presentation of the radiation beamfrom a variety of angles with respect to the patient without requiringuncomfortable or inconvenient positioning of the patient themselves.However, as the gantry structure is relatively large (on the order ofseveral meters), massive, and made out of non-strictly rigid materials,there is inevitably some degree of structural flex/distortion andnon-repeatable mechanical tolerance as the nozzle revolves about thegantry. Further, the nozzle may be configured as an elongate distributedmass that is also not strictly rigid such that the distal emissions endof the nozzle can flex to some degree, for example as the nozzle movesfrom an overhead vertical position to a horizontal, sidewayspresentation of the beam. Accurate identification of the precise nozzleposition can also be complicated by a cork screwing with the gantry.

Similarly, the patient may be placed on a supportive pod or table and itmay be connected to a patient positioning apparatus, both of which aresubject to some degree of mechanical flex under gravity load, as well asmechanical tolerances at moving joints that are not necessarilyconsistent throughout the range of possible patient postures. While itis possible to estimate and measure certain of these variations, as theyare typically variable and non-repeatable, it remains a significantchallenge to repeatedly position a patient consistently over multipletreatment sessions in both location and pose to tight accuracy limits,such as to millimeter or less accuracy on a predictive basis. Thus, theknown way to address gantry and patient table misalignment is tore-register the patient before treatment. This is undesirable as thepatient is exposed to additional x-ray radiation for the imaging andoverall patient throughput is reduced by the added latency of there-registration.

From the foregoing it will be understood that there is a need forincreasing the accuracy and speed of the patient registration process.There is also a need for reducing iteratively imaging and reorientingthe patient to achieve a desired pose. There is also a need for a systemthat accounts for variable and unpredictable position errors to increasethe accuracy of patient registration and alignment with a radiationtherapy delivery system.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a patient alignment system thatexternally measures and provides corrective feedback for variations ordeviations from nominal position and orientation between the patient anda delivered therapeutic radiation beam. The alignment system can readilyaccommodate variable and unpredictable mechanical tolerances andstructural flex of both fixed and movable components of the radiationtherapy system. The patient alignment system reduces the need forimaging the patient between treatment fractions and decreases thelatency of the registration process, thus increasing patient throughput.

Other embodiments comprise a radiation therapy delivery systemcomprising a gantry, a patient fixation device configured to secure apatient with respect to the patient fixation device, a patientpositioner interconnected to the patient fixation device so as toposition the patient fixation device along translational and rotationalaxes within the gantry, a radiation therapy nozzle interconnected to thegantry and selectively delivering radiation therapy along a beam axis, aplurality of external measurement devices which obtain positionmeasurements of at least the patient fixation device and the nozzle, anda controller which receives the position measurements of at least thepatient fixation device and the nozzle and provides control signals tothe patient positioner to position the patient in a desired orientationwith respect to the beam axis.

Another embodiment comprises a patient positioning system for aradiation therapy system having a plurality of components that aresubject to movement, the positioning system comprising a plurality ofexternal measurement devices arranged to obtain position measurements ofthe plurality of components so as to provide location information, amovable patient support configured to support a patient substantiallyfixed in position with respect to the patient support and controllablyposition the patient in multiple translational and rotational axes, anda controller receiving information from the plurality of externalmeasurement devices and providing movement commands to the movablepatient support to align the patient in a desired pose such that thepositioning system compensates for movement of the plurality ofcomponents.

Further embodiments include a method of registering and positioning apatient for delivery of therapy with a system having a plurality ofcomponents subject to movement, the method comprising the steps ofpositioning a patient in an initial treatment pose with a controllablepatient positioner, externally measuring the location of selected pointsof the plurality of components, determining a difference vector betweenthe observed initial patient pose and a desired patient pose, andproviding movement commands to the patient positioner to bring thepatient to the desired patient pose.

Yet another embodiment comprises a positioning system for use with aradiation treatment facility wherein the radiation treatment facilityhas a plurality of components that includes a source of particles and anozzle from which the particles are emitted, wherein the nozzle ismovable with respect to the patient to facilitate delivery of theparticles to a selected region of the patient via a plurality ofdifferent paths, the positioning system comprising a patient positionerthat receives the patient wherein the patient positioner is movable soas to orient the patient with respect to the nozzle to facilitatedelivery of the particles in the selected region of the patient, amonitoring system that images at least one component of the radiationtreatment facility in proximity to the patient positioner, wherein themonitoring system develops a treatment image indicative of theorientation of the at least one component with respect to the patientprior to treatment, and a control system that controls delivery ofparticles to the patient wherein the control system receives signalsindicative of the treatment to be performed, the signals including adesired orientation of the at least one component when the particles areto be delivered to the patient, wherein the control system furtherreceives the treatment image and the control system evaluates thetreatment image to determine an actual orientation of the at least onecomponent prior to treatment and wherein the control system compares theactual orientation of the at least one component prior to treatment tothe desired orientation of the at least one component and, if the actualorientation does not meet a pre-determined criteria for correspondencewith the desired orientation, the control system sends signals to thepatient positioner to move the patient positioner such that the actualorientation more closely corresponds to the desired orientation duringdelivery of the particles.

These and other objects and advantages of the invention will become moreapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A schematic diagram of one embodiment of a radiation therapy system witha patient positioning system in a first orientation is shown in FIG. 1Aand in a second orientation in FIG. 1B;

FIG. 2A illustrates one embodiment of retractable imagers in an extendedposition and FIG. 2B illustrates the imagers in a retracted position;

FIG. 3 illustrates one embodiment of a patient positioner to which apatient pod can be attached;

FIGS. 4A-4E illustrate various position error sources of one embodimentof a radiation therapy system;

FIG. 5 is a flow chart of one embodiment of a method of determining theposition and orientation of objects in a radiation therapy environment;

FIG. 6 illustrates one embodiment of external measurement devices for aradiation therapy system;

FIG. 7 illustrates further embodiments of external measurement devicesfor a radiation therapy system;

FIG. 8 is a block diagram of one embodiment of a precision patientpositioning system of a radiation therapy system;

FIG. 9 is a block diagram of one embodiment of an external measurementand 6D coordination system of the patient positioning system;

FIG. 10 is a block diagram of a patient registration module of thepatient positioning system;

FIG. 11 is a block diagram of a path planning module of a motion controlmodule of the patient positioning system;

FIG. 12 is a block diagram of an active collision avoidance module ofthe motion control module of the patient positioning system;

FIG. 13 is a block diagram of one embodiment of the collision avoidancemodule and a motion sequence coordinator of a motion control module; and

FIG. 14 is a flow chart of the operation of one embodiment of a methodof positioning a patient and delivering radiation therapy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made to the drawings wherein like referencedesignators refer to like parts throughout. FIGS. 1A and 1B illustrateschematically first and second orientations of one embodiment of aradiation therapy system 100, such as based on the proton therapy systemcurrently in use at Loma Linda University Medical Center in Loma Linda,Calif. and as described in U.S. Pat. No. 4,870,287 of Sep. 26, 1989which is incorporated herein in its entirety by reference. The radiationtherapy system 100 is designed to deliver therapeutic radiation doses toa target region within a patient for treatment of malignancies or otherconditions from one or more angles or orientations with respect to thepatient. The system 100 includes a gantry 102 which includes a generallyhemispherical or frustoconical support frame for attachment and supportof other components of the radiation therapy system 100. Additionaldetails on the structure and operation of embodiments of the gantry 102may be found in U.S. Pat. No. 4,917,344 and U.S. Pat. No. 5,039,057,both of which are incorporated herein in their entirety by reference.

The system 100 also comprises a nozzle 104 which is attached andsupported by the gantry 102 such that the gantry 102 and nozzle 104 mayrevolve relatively precisely about a gantry isocenter 120, but subjectto corkscrew, sag, and other distortions from nominal. The system 100also comprises a radiation source 106 delivering a radiation beam alonga radiation beam axis 140, such as a beam of accelerated protons. Theradiation beam passes through and is shaped by an aperture 110 to definea therapeutic beam delivered along a delivery axis 142. The aperture 110is positioned on the distal end of the nozzle 104 and the aperture 110may preferably be specifically configured for a patient's particularprescription of therapeutic radiation therapy. In certain applications,multiple apertures 110 are provided for different treatment fractions.

The system 100 also comprises one or more imagers 112 which, in thisembodiment, are retractable with respect to the gantry 102 between anextended position as illustrated in FIG. 2A and a retracted position asillustrated in FIG. 2B. The imager 112 in one implementation comprises acommercially available solid-state amorphous silicon x-ray imager whichcan develop image information such as from incident x-ray radiation thathas passed through a patient's body. The retractable aspect of theimager 112 provides the advantage of withdrawing the imager screen fromthe delivery axis 142 of the radiation source 106 when the imager 112 isnot needed thereby providing additional clearance within the gantry 102enclosure as well as placing the imager 112 out of the path ofpotentially harmful emissions from the radiation source 106 therebyreducing the need for shielding to be provided to the imager 112.

The system 100 also comprises corresponding one or more x-ray sources130 which selectively emit appropriate x-ray radiation along one or morex-ray source axes 144 so as to pass through interposed patient tissue togenerate a radiographic image of the interposed materials via the imager112. The particular energy, dose, duration, and other exposureparameters preferably employed by the x-ray source(s) 130 for imagingand the radiation source 106 for therapy will vary in differentapplications and will be readily understood and determined by one ofordinary skill in the art.

In this embodiment, at least one of the x-ray sources 130 ispositionable such that the x-ray source axis 144 can be positioned so asto be nominally coincident with the delivery axis 142. This embodimentprovides the advantage of developing a patient image for registrationfrom a perspective which is nominally identical to a treatmentperspective. This embodiment also includes the aspect that a firstimager 112 and x-ray source 130 pair and a second imager 112 and x-raysource 130 pair are arranged substantially orthogonal to each other.This embodiment provides the advantage of being able to obtain patientimages in two orthogonal perspectives to increase registration accuracyas will be described in greater detail below. The imaging system can besimilar to the systems described in U.S. Pat. Nos. 5,825,845 and5,117,829 which are hereby incorporated by reference.

The system 100 also comprises a patient positioner 114 (FIG. 3) and apatient pod 116 which is attached to a distal or working end of thepatient positioner 114. The patient positioner 114 is adapted to, uponreceipt of appropriate movement commands, position the patient pod 116in multiple translational and rotational axes and preferably is capableof positioning the patient pod 116 in three orthogonal translationalaxes as well as three orthogonal rotational axes so as to provide a fullsix degree freedom of motion to placement of the patient pod 116.

The patient pod 116 is configured to hold a patient securely in place inthe patient pod 116 so to as substantially inhibit any relative movementof the patient with respect to the patient pod 116. In variousembodiments, the patient pod 116 comprises expandable foam, bite blocks,and/or fitted facemasks as immobilizing devices and/or materials. Thepatient pod 116 is also preferably configured to reduce difficultiesencountered when a treatment fraction indicates delivery at an edge ortransition region of the patient pod 116. Additional details ofpreferred embodiments of the patient positioner 114 and patient pod 116can be found in the commonly assigned application (serial number10/917,022, filed Aug. 12, 2004) entitled “Modular Patient SupportSystem” filed concurrently herewith and which is incorporated herein inits entirety by reference.

As previously mentioned, in certain applications of the system 100,accurate relative positioning and orientation of the therapeutic beamdelivery axis 142 provided by the radiation source 106 with targettissue within the patient as supported by the patient pod 116 andpatient positioner 114 is an important goal of the system 100, such aswhen comprising a proton beam therapy system. However, as previouslymentioned, the various components of the system 100, such as the gantry102, the nozzle 104, radiation source 106, the imager(s) 112, thepatient positioner 114, the patient pod 116, and x-ray source(s) 130 aresubject to certain amounts of structural flex and movement tolerancesfrom a nominal position and orientation which can affect accuratedelivery of the beam to that patient.

FIGS. 1A and 1B illustrate different arrangements of certain componentsof the system 100 and indicate by the broken arrows both translationaland rotational deviations from nominal that can occur in the system 100.For example, in the embodiment shown in FIG. 1A, the nozzle 104 andfirst imager 112 extend substantially horizontally and are subject tobending due to gravity, particularly at their respective distal ends.The second imager 112 is arranged substantially vertically and is notsubject to the horizontal bending of the first imager 112. FIG. 1Billustrates the system 100 in a different arrangement rotatedapproximately 45° counterclockwise from the orientation of FIG. 1A. Inthis orientation, both of the imagers 112 as well as the nozzle 104 aresubject to bending under gravity, but to a different degree than in theorientation illustrated in FIG. 1A. The movement of the gantry 102between different orientations, such as is illustrated in FIGS. 1A and1B also subjects components of the system 100 to mechanical tolerancesat the moving surfaces. As these deviations from nominal are at leastpartially unpredictable, non-repeatable, and additive, correcting forthe deviations on a predictive basis is extremely challenging and limitsoverall alignment accuracy. It will be appreciated that these deviationsfrom the nominal orientation of the system are simply exemplary and thatany of a number of sources of error can be addressed by the systemdisclosed herein without departing from the spirit of the presentinvention.

FIGS. 4A-4E illustrate in greater detail embodiments of potentialuncertainties or errors which can present themselves upon procedures foralignment of, for example, the nozzle 104 and the target tissue of thepatient at an isocenter 120. FIGS. 4A-4E illustrate these sources ofuncertainty or error with reference to certain distances and positions.It will be appreciated that the sources of error described are simplyillustrative of the types of errors addressed by the system 100 of theillustrated embodiments and that the system 100 described is capable ofaddressing additional errors. In this embodiment, a distance SAD isdefined as a source to axis distance from the radiation source 106 tothe rotation axis of the gantry, which ideally passes through theisocenter 120. For purposes of explanation and appreciation of relativescale and distances, in this embodiment, SAD is approximately equal to2.3 meters.

FIG. 4A illustrates that one of the potential sources of error is asource error where the true location of the radiation source 106 issubject to offset from a presumed or nominal location. In thisembodiment, the therapeutic radiation beam as provided by the radiationsource 106 passes through two transmission ion chambers (TIC) whichserve to center the beam. These are indicated as TIC 1 and TIC 3 andthese are also affixed to the nozzle 104. The source error can arisefrom numerous sources including movement of the beam as observed on TIC1 and/or TIC 3, error in the true gantry 102 rotational angle, and errordue to “egging” or distortion from round of the gantry 102 as itrotates. FIG. 4A illustrates source error comprising an offset of thetrue position of the radiation source 106 from a presumed or nominallocation and the propagation of the radiation beam across the SADdistance through the aperture 110 providing a corresponding error atisocenter 120.

FIG. 4B illustrates possible error caused by TIC location error, whereTIC 1, the radiation source 106, and TIC 3 are offset from an ideal beamaxis passing through the nominal gantry isocenter 120. As the errorsillustrated by FIG. 4A and 4B are assumed random and uncorrelated, theycan be combined in quadrature and projected through an assumed nominalcenter of the aperture 110 to establish a total error contribution dueto radiation source 106 error projected to the isocenter 120. In thisembodiment, before corrective measures are taken (as described ingreater detail below), the radiation source error can range fromapproximately ±0.6 mm to ±0.4 mm.

FIG. 4C illustrates error or uncertainty due to position of the aperture110. The location of the radiation source 106 is assumed nominal;however, error or uncertainty is introduced both by tolerance stack-up,skew, and flex of the nozzle 104 as well as manufacturing tolerances ofthe aperture 110 itself. Again, as projected from the radiation source106 across the distance SAD to the nominal isocenter 120, a beamdelivery aiming point (BDAP) error is possible between a presumednominal BDAP and an actual BDAP. In this embodiment, this BDAP errorarising from error in the aperture 110 location ranges fromapproximately ±1.1 mm to ±1.5 mm.

The system 100 is also subject to error due to positioning of theimager(s) 112 as well as the x-ray source(s) 130 as illustrated in FIGS.4D and 4E. FIG. 4D illustrates the error due to uncertainty in theimager(s) 112 position with the position of the corresponding x-raysource(s) 130 assumed nominal. As the emissions from the x-ray source130 pass through the patient assumed located substantially at isocenter120 and onward to the imager 112, this distance may be different thanthe SAD distance and in this embodiment is approximately equal to 2.2meters. Error or uncertainty in the true position of an imager 112 canarise from lateral shifts in the true position of the imager 112, errorsdue to axial shifting of the imager 112 with respect to thecorresponding x-ray source 130, as well as errors in registration ofimages obtained by imager 112 to the DRRs. In this embodiment, beforecorrection, the errors due to each imager 112 are approximately ±0.7 mm.

Similarly, FIG. 4E illustrates errors due to uncertainty in positioningof the x-ray source(s) 130 with the position of the correspondingimager(s) 112 assumed nominal. Possible sources of error due to thex-ray source 130 include errors due to initial alignment of the x-raysource 130, errors arising from movement of the x-ray source 130 intoand out of the beam line, and errors due to interpretation of sags andrelative distances of TIC 1 and TIC 3. These errors are also assumedrandom and uncorrelated or independent and are thus added in quadratureresulting, in this embodiment, in error due to each x-ray source 130 ofapproximately ±0.7 mm.

As these errors are random and independent and uncorrelated and thuspotentially additive, in this embodiment the system 100 also comprises aplurality of external measurement devices 124 to evaluate and facilitatecompensating for these errors. In one embodiment, the system 100 alsocomprises monuments, such as markers 122, cooperating with the externalmeasurement devices 124 as shown in FIGS. 2A, 2B, 6 and 7. The externalmeasurement devices 124 each obtain measurement information about thethree-dimensional position in space of one or more components of thesystem 100 as indicated by the monuments as well as one or more fixedlandmarks 132 also referred to herein as the “world” 132.

In this embodiment, the external measurement devices 124 comprisecommercially available cameras, such as CMOS digital cameras withmegapixel resolution and frame rates of 200-1000Hz, which independentlyobtain optical images of objects within a field of view 126, which inthis embodiment is approximately 85° horizontally and 70° vertically.The external measurement devices 124 comprising digital cameras arecommercially available, for example as components of the Vicon Trackersystem from Vicon Motion Systems Inc. of Lake Forrest, Calif. However,in other embodiments, the external measurement devices 124 can compriselaser measurement devices and/or radio location devices in addition toor as an alternative to the optical cameras of this embodiment.

In this embodiment, the markers 122 comprise spherical, highlyreflective landmarks which are fixed to various components of the system100. In this embodiment, at least three markers 122 are fixed to eachcomponent of the system 100 of interest and are preferably placedasymmetrically, e.g. not equidistant from a centerline nor evenly oncorners, about the object. The external measurement devices 124 arearranged such that at least two external measurement devices 124 have agiven component of the system 100 and the corresponding markers 122 intheir field of view and in one embodiment a total of ten externalmeasurement devices 124 are provided. This aspect provides the abilityto provide binocular vision to the system 100 to enable the system 100to more accurately determine the location and orientation of componentsof the system 100. The markers 122 are provided to facilitaterecognition and precise determination of the position and orientation ofthe objects to which the markers 122 are affixed, however in otherembodiments, the system 100 employs the external measurement devices 124to obtain position information based on monuments comprisingcharacteristic outer contours of objects, such as edges or corners,comprising the system 100 without use of the external markers 122.

FIG. 5 illustrates one embodiment of determining the spatial positionand angular orientation of a component of the system 100. As thecomponent(s) of interest can be the gantry 102, nozzle 104, aperture110, imager 112, world 132 or other components, reference will be madeto a generic “object”. It will be appreciated that the process describedfor the object can proceed in parallel or in a series manner formultiple objects. Following a start state, in state 150 the system 100calibrates the multiple external measurement devices 124 with respect toeach other and the world 132. In the calibration state, the system 100determines the spatial position and angular orientation of each externalmeasurement device 124. The system 100 also determines the location ofthe world 132 which can be defined by a dedicated L-frame and can definea spatial origin or frame-of-reference of the system 100. The world 132can, of course, comprise any component or structure that issubstantially fixed within the field of view of the external measurementdevices 124. Hence, structures that are not likely to move or deflect asa result of the system 100 can comprise the world 132 or point ofreference for the external measurement devices 124.

A wand, which can include one or more markers 122 is moved within thefields of view 126 of the external measurement devices 124. As theexternal measurement devices 124 are arranged such that multipleexternal measurement devices 124 (in this embodiment at least two) havean object in the active area of the system 100 in their field of view126 at any given time, the system 100 correlates the independentlyprovided location and orientation information from each externalmeasurement device 124 and determines corrective factors such that themultiple external measurement devices 124 provide independent locationand orientation information that is in agreement following calibration.The particular mathematical steps to calibrate the external measurementdevices 124 are dependent on their number, relative spacing, geometricalorientations to each other and the world 132, as well as the coordinatesystem used and can vary among particular applications, however will beunderstood by one of ordinary skill in the art. It will also beappreciated that in certain applications, the calibration state 150would need to be repeated if one or more of the external measurementdevices 124 or world 132 is moved following calibration.

Following the calibration state 150, in state 152 multiple externalmeasurement devices 124 obtain an image of the object(s) of interest.From the images obtained in state 152, the system 100 determines acorresponding direction vector 155 to the object from each correspondingexternal measurement device 124 which images the object in state 154.This is illustrated in FIG. 6 as vectors 155 a-d corresponding to theexternal measurement devices 124 a-d which have the object in theirrespective fields of view 126. Then, in state 156, the system 100calculates the point in space where the vectors 155 (FIG. 6) determinedin state 154 intersect. State 156 thus returns a three-dimensionallocation in space, with reference to the world 132, for the objectcorresponding to multiple vectors intersecting at the location. As theobject has been provided with three or more movements or markers 122,the system 100 can also determine the three-dimensional angularorientation of the object by evaluating the relative locations of theindividual markers 122 associated with the object. In thisimplementation, the external measurement devices 124 comprise cameras,however, any of a number of different devices can be used to image,e.g., determine the location, of the monuments without departing fromthe spirit of the present invention. In particular, devices that emit orreceive electromagnetic or audio energy including visible andnon-visible wavelength energy and ultra-sound can be used to image ordetermine the location of the monuments.

The location and orientation information determined for the object isprovided in state 160 for use in the system 100 as described in greaterdetail below. In one embodiment, the calibration state 150 can beperformed within approximately one minute and allows the system 100 todetermine the object's location in states 152, 154, 156, and 160 towithin 0.1 mm and orientation to within 0.15° with a latency of no morethan 10 ms. As previously mentioned, in other embodiments, the externalmeasurement devices 124 can comprise laser measurement devices,radio-location devices or other devices that can determine direction toor distance from the external measurement devices 124 in addition to oras an alternative to the external measurement devices 124 describedabove. Thus, in certain embodiments a single external measurement device124 can determine both range and direction to the object to determinethe object location and orientation. In other embodiments, the externalmeasurement devices 124 provide only distance information to the objectand the object's location in space is determined by determining theintersection of multiple virtual spheres centered on the correspondingexternal measurement devices 124.

In certain embodiments, the system 100 also comprises one or more localposition feedback devices or resolvers 134 (See, e.g., FIG. 1). Thelocal feedback devices or resolvers 134 are embodied within or incommunication with one or more components of the system 100, such as thegantry 102, the nozzle 104, the radiation source 106, the aperture 110,the imager(s) 112, patient positioner 114, patient pod 116, and/or world132. The local feedback devices 134 provide independent positioninformation relating to the associated component of the system 100. Invarious embodiments, the local feedback devices 134 comprise rotaryencoders, linear encoders, servos, or other position indicators that arecommercially available and whose operation is well understood by one ofordinary skill in the art. The local feedback devices 134 provideindependent position information that can be utilized by the system 100in addition to the information provided by the external measurementdevices 124 to more accurately position the patient.

The system 100 also comprises, in this embodiment, a precision patientalignment system 200 which employs the location information provided instate 160 for the object(s). As illustrated in FIG. 8, the patientalignment system 200 comprises a command and control module 202communicating with a 6D system 204, a patient registration module 206,data files 210, a motion control module 212, a safety module 214, and auser interface 216. The patient alignment system 200 employs locationinformation provided by the 6D system 204 to more accurately registerthe patient and move the nozzle 104 and the patient positioner 114 toachieve a desired treatment pose as indicated by the prescription forthe patient provided by the data files 210.

In this embodiment, the 6D system 204 receives position data from theexternal measurement devices 124 and from the resolvers 134 relating tothe current location of the nozzle 104, the aperture 110, the imager112, the patient positioner 114, and patient pod 116, as well as thelocation of one or more fixed landmarks 132 indicated in FIG. 9 as theworld 132. The fixed landmarks, or world, 132 provide a non-movingorigin or frame of reference to facilitate determination of the positionof the moving components of the radiation therapy system 100. Thislocation information is provided to a primary 6D position measurementsystem 220 which then uses the observed data from the externalmeasurement devices 124 and resolvers 134 to calculate position andorientation coordinates of these five components and origin in a firstreference frame. This position information is provided to a 6Dcoordination module 222 which comprises a coordinate transform module224 and an arbitration module 226. The coordinate transform module 224communicates with other modules of the patient alignment system 200,such as the command and control module 202 and the motion control withpath planning and collision avoidance module 212.

Depending on the stage of the patient registration and therapy deliveryprocess, other modules of the patient alignment system 200 can submitcalls to the 6D system 204 for a position request of the currentconfiguration of the radiation therapy system 100. Other modules of thepatient alignment system 200 can also provide calls to the 6D system 204such as a coordinate transform request. Such a request typically willinclude submission of location data in a given reference frame, anindication of the reference frame in which the data is submitted and adesired frame of reference which the calling module wishes to have theposition data transformed into. This coordinate transform request issubmitted to the coordinate transform module 224 which performs theappropriate calculations upon the submitted data in the given referenceframe and transforms the data into the desired frame of reference andreturns this to the calling module of the patient alignment system 200.

For example, the radiation therapy system 100 may determine thatmovement of the patient positioner 114 is indicated to correctlyregister the patient. For example, a translation of plus 2 mm along anx-axis, minus 1.5 mm along a y-axis, no change along a z-axis, and apositive 1° rotation about a vertical axis is indicated. This data wouldbe submitted to the coordinate transform module 224 which would thenoperate upon the data to return corresponding movement commands to thepatient positioner 114. The exact coordinate transformations will varyin specific implementations of the system 100 depending, for example, onthe exact configuration and dimensions of the patient positioner 114 andthe relative position of the patient positioner 114 with respect toother components of the system 100. However, such coordinate transformscan be readily determined by one of ordinary skill in the art for aparticular application.

The arbitration module 226 assists in operation of the motion controlmodule 212 by providing specific object position information uponreceipt of a position request. A secondary position measurement system230 provides an alternative or backup position measurement function forthe various components of the radiation therapy system 100. In oneembodiment, the secondary position measurement system 230 comprises aconventional positioning functionality employing predicted positioninformation based on an initial position and commanded moves. In oneembodiment, the primary position measurement system 220 receivesinformation from the external measurement devices 124 and the secondaryposition measurement system 230 receives independent positioninformation from the resolvers 134. It will generally be preferred thatthe 6D measurement system 220 operate as the primary positioning systemfor the previously described advantages of positioning accuracy andspeed.

FIG. 10 illustrates in greater detail the patient registration module206 of the patient alignment system 200. As previously described, the 6Dsystem 204 obtains location measurements of various components of theradiation therapy system 100, including the table or patient pod 116 andthe nozzle 104 and determines position coordinates of these variouscomponents and presents them in a desired frame of reference. The datafiles 210 provide information relating to the patient's treatmentprescription, including the treatment plan and CT data previouslyobtained at a planning or prescription session. This patient's data canbe configured by a data converter 232 to present the data in a preferredformat. The imager 112 also provides location information to the 6Dsystem 204 as well as to an image capture module 236. The image capturemodule 236 receives raw image data from the imager 112 and processesthis data, such as with filtering, exposure correction, scaling, andcropping to provide corrected image data to a registration algorithm241.

In this embodiment, the CT data undergoes an intermediate processingstep via a transgraph creation module 234 to transform the CT data intotransgraphs which are provided to the registration algorithm 241. Thetransgraphs are an intermediate data representation and increase thespeed of generation of DRRs. The registration algorithm 241 uses thetransgraphs, the treatment plan, the current object position dataprovided by the 6D system 204 and the corrected image data from theimager(s) 112 to determine a registered pose which information isprovided to the command and control module 202. The registrationalgorithm 241 attempts to match either as closely as possible or towithin a designated tolerance the corrected image data from the imager112 with an appropriate DRR to establish a desired pose or to registerthe patient. The command and control module 202 can evaluate the currentregistered pose and provide commands or requests to induce movement ofone or more of the components of the radiation therapy system 100 toachieve this desired pose. Additional details for a suitableregistration algorithm may be found in the published doctoraldissertation of David A. LaRose of May 2001 submitted to Carnegie MellonUniversity entitled “Iterative X-ray/CT Registration Using AcceleratedVolume Rendering” which is incorporated herein in its entirety byreference.

FIGS. 11-13 illustrate embodiments with which the system 100 performsthis movement. FIG. 11 illustrates that the command and control module202 has provided a call for movement of one or more of the components ofthe radiation therapy system 100. In state 238, the motion controlmodule 212 retrieves a current position configuration from the 6D system204 and provides this with the newly requested position configuration toa path planning module 240. The path planning module 240 comprises alibrary of three-dimensional model data which represent positionenvelopes defined by possible movement of the various components of theradiation therapy system 100. For example, as previously described, theimager 112 is retractable and a 3D model data module 242 indicates theenvelope or volume in space through which the imager 112 can movedepending on its present and end locations.

The path planning module 240 also comprises an object movement simulator244 which receives data from the 3D model data module 242 and cancalculate movement simulations for the various components of theradiation therapy system 100 based upon this data. This object movementsimulation module 244 preferably works in concert with a collisionavoidance module 270 as illustrated in FIG. 12. FIG. 12 againillustrates one embodiment of the operation of the 6D system 204 whichin this embodiment obtains location measurements of the aperture 110,imager 112, nozzle 104, patient positioner and patient pod 114 and 116as well as the fixed landmarks or world 132. FIG. 12 also illustratesthat, in this embodiment, local feedback is gathered from resolvers 134corresponding to the patient positioner 114, the nozzle 104, the imager112, and the angle of the gantry 102.

This position information is provided to the collision avoidance module270 which gathers the object information in an object position datalibrary 272. This object data is provided to a decision module 274 whichevaluates whether the data is verifiable. In certain embodiments, theevaluation of the module 274 can investigate possible inconsistencies orconflicts with the object position data from the library 272 such asout-of-range data or data which indicates, for example, that multipleobjects are occupying the same location. If a conflict or out-of-rangecondition is determined, e.g., the result of the termination module 274is negative, a system halt is indicated in state 284 to inhibit furthermovement of components of the radiation therapy system 100 and furtherproceeds to a fault recovery state 286 where appropriate measures aretaken to recover or correct the fault or faults. Upon completion of thefault recovery state 286, a reset state 290 is performed followed by areturn to the data retrieval of the object position data library inmodule 272.

If the evaluation of state 274 is affirmative, a state 276 follows wherethe collision avoidance module 270 calculates relative distances alongcurrent and projected trajectories and provides this calculatedinformation to an evaluation state 280 which determines whether one ormore of the objects or components of the radiation therapy system 100are too close. If the evaluation of stage 280 is negative, e.g., thatthe current locations and projected trajectories do not present acollision hazard, a sleep or pause state 282 follows during whichmovement of the one or more components of the radiation therapy system100 is allowed to continue as indicated and proceeds to a recursivesequence through modules 272, 274, 276, 280, and 282 as indicated.

However, if the results of the evaluation state 280 are affirmative,e.g., that either one or more of the objects are too close or that theirprojected trajectories would bring them into collision, the system haltof state 284 is implemented with the fault recovery and reset states 286and 290, following as previously described. Thus, the collisionavoidance module 270 allows the radiation therapy system 100 toproactively evaluate both current and projected locations and movementtrajectories of movable components of the system 100 to mitigatepossible collisions before they occur or are even initiated. This isadvantageous over systems employing motion stops triggered, for example,by contact switches which halt motion upon activation of stop or contactswitches, which by themselves may be inadequate to prevent damage to themoving components which can be relatively large and massive havingsignificant inertia, or to prevent injury to a user or patient of thesystem.

Assuming that the object movement simulation module 244 as cooperatingwith the collision avoidance module 270 indicates that the indicatedmovements will not pose a collision risk, the actual movement commandsare forwarded to a motion sequence coordinator module 246 whichevaluates the indicated movement vectors of the one or more componentsof the radiation therapy system 100 and sequences these movements via,in this embodiment, five translation modules. In particular, thetranslation modules 250, 252, 254, 260, and 262 translate indicatedmovement vectors from a provided reference frame to a command referenceframe appropriate to the patient positioner 114, the gantry 102, thex-ray source 130, the imager 112, and the nozzle 104, respectively.

As previously mentioned, the various moveable components of theradiation therapy system 100 can assume different dimensions and besubject to different control parameters and the translation modules 250,252, 254, 260, and 262 interrelate or translate a motion vector in afirst frame of reference into the appropriate reference frame for thecorresponding component of the radiation therapy system 100. Forexample, in this embodiment the gantry 102 is capable of clockwise andcounterclockwise rotation about an axis whereas the patient positioner114 is positionable in six degrees of translational and rotationalmovement freedom and thus operates under a different frame of referencefor movement commands as compared to the gantry 102. By having theavailability of externally measured location information for the variouscomponents of the radiation therapy system 100, the motion sequencecoordinator module 246 can efficiently plan the movement of thesecomponents in a straightforward, efficient and safe manner.

FIG. 14 illustrates a workflow or method 300 of one embodiment ofoperation of the radiation therapy system 100 as provided with thepatient alignment system 200. From a start state 302, follows anidentification state 304 wherein the particular patient and treatmentportal to be provided is identified. This is followed by a treatmentprescription retrieval state 306 and the identification and treatmentprescription retrieval of states 304 and 306 can be performed via theuser interface 216 and accessing the data files of module 210. Thepatient is then moved to an imaging position in state 310 by enteringinto the patient pod 116 and actuation of the patient positioner 114 toposition the patient pod 116 securing the patient in the approximateposition for imaging. The gantry 102, imager(s) 112, and radiationsource(s) 130 are also moved to an imaging position in state 312 and instate 314 the x-ray imaging axis parameters are determined as previouslydescribed via the 6D system 204 employing the external measurementdevices 124, cooperating markers 122, and resolvers 134.

In state 316, a radiographic image of the patient is captured by theimager 112 and corrections can be applied as needed as previouslydescribed by the module 236. In this embodiment, two imagers 112 andcorresponding x-ray sources 130 are arranged substantiallyperpendicularly to each other. Thus, two independent radiographic imagesare obtained from orthogonal perspectives. This aspect provides morecomplete radiographic image information than from a single perspective.It will also be appreciated that in certain embodiments, multipleimaging of states 316 can be performed for additional data. Anevaluation is performed in state 320 to determine whether theradiographic image acquisition process is complete and the determinationof this decision results either in the negative case with continuationof the movement of state 312, the determination of state 314 and thecapture of state 316 as indicated or, when affirmative, followed bystate 322.

In state 322, external measurements are performed by the 6D system 204as previously described to determine the relative positions andorientations of the various components of the radiation therapy system100 via the patient registration module 206 as previously described. Instate 324, motion computations are made as indicated to properly alignthe patient in the desired pose.

While not necessarily required in each instance of treatment delivery,this embodiment illustrates that in state 326 some degree of gantry 102movement is indicated to position the gantry 102 in a treatment positionas well as movement of the patient, such as via the patient positioner114 in state 330 to position the patient in the indicated pose.Following these movements, state 332 again employs the 6D system 204 toexternally measure and in state 334 to compute and analyze the measuredposition to determine in state 336 whether the desired patient pose hasbeen achieved within the desired tolerance. If adequately accurateregistration and positioning of the patient has not yet been achieved,state 340 follows where a correction vector is computed and transformedinto the appropriate frame of reference for further movement of thegantry 102 and/or patient positioner 114. If the decision of state 336is affirmative, e.g., that the patient has been satisfactorilypositioned in the desired pose, the radiation therapy fraction isenabled in state 342 in accordance with the patient's prescription. Forcertain patient prescriptions, it will be understood that the treatmentsession may indicate multiple treatment fractions, such as treatmentfrom a plurality of orientations and that appropriate portions of themethod 300 may be iteratively repeated for multiple prescribed treatmentfractions. However, for simplicity of illustration, a single iterationis illustrated in FIG. 14. Thus, following the treatment delivery ofstate 342, a finished state 344 follows which may comprise thecompletion of treatment for that patient for the day or for a givenseries of treatments.

Thus, the radiation therapy system 100 with the patient alignment system200, by directly measuring movable components of the system 100, employsa measured feedback to more accurately determine and control thepositioning of these various components. A particular advantage of thesystem 100 is that the patient can be more accurately registered at atreatment delivery session than is possible with known systems andwithout an iterative sequence of radiographic imaging, repositioning ofthe patient, and subsequent radiographic imaging and data analysis. Thisoffers the significant advantage both of more accurately delivering thetherapeutic radiation, significantly decreasing the latency of theregistration, imaging and positioning processes and thus increasing thepossible patient throughput as well as reducing the exposure of thepatient to x-ray radiation during radiographic imaging by reducing theneed for multiple x-ray exposures during a treatment session.

Although the preferred embodiments of the present invention have shown,described and pointed out the fundamental novel features of theinvention as applied to those embodiments, it will be understood thatvarious omissions, substitutions and changes in the form of the detailof the device illustrated may be made by those skilled in the artwithout departing from the spirit of the present invention.Consequently, the scope of the invention should not be limited to theforegoing description but is to be defined by the appended claims.

1. A radiation therapy delivery system comprising: a gantry; a patientfixation device configured to secure a patient with respect to thepatient fixation device; a movable patient positioner connected to thepatient fixation device so as to position the patient fixation devicealong translational and rotational axes; a radiation therapy nozzleconnected to the gantry and adapted to selectively deliver radiationtherapy along a beam axis; a plurality of external measurement deviceswhich obtain position measurements of at least the patient fixationdevice and the nozzle with respect to a substantially fixed frame ofreference; and a controller which receives the relative positionmeasurements of at least the patient fixation device and the nozzle andprovides control signals to the patient positioner to position thepatient in a desired orientation with respect to the beam axis.
 2. Thesystem of claim 1, wherein the external measurement devices obtainoptical images of at least the patient fixation device and the nozzle toobtain the relative position measurements.
 3. The system of claim 1,further comprising a plurality of markers fixed to at least the patientfixation device and the nozzle wherein the external measurement devicesobtain position measurements of the markers.
 4. The system of claim 1,further comprising at least one local position feedback device inoperative communication with at least one of the gantry and patientpositioner wherein the at least one local position feedback deviceprovides position data to the controller indicative of the relativeposition of the at least one of the gantry and patient positioner. 5.The system of claim 1, wherein the system further comprises a patientimaging system which obtains an image of the patient and determines therelative location of a patient target isocenter with respect to thenozzle.
 6. The system of claim 5, wherein the system positions thepatient in an initial position and, after imaging the patient,determines a correction vector and provides control signals to move thepatient positioner and the patient target isocenter into a desiredpatient pose with respect to the nozzle.
 7. The system of claim 5,wherein the patient imaging system comprises at least one x-ray sourceand at least one solid state x-ray imager.
 8. The system of claim 5,wherein the patient imaging system is movable in and out of the beamaxis.
 9. The system of claim 1, wherein the nozzle is interconnected tothe gantry so as to be revolvable about a gantry isocenter.
 10. Thesystem of claim 1, wherein the controller determines an orientation andposition difference vector in the substantially fixed frame of referenceand transforms the difference vector into at least one correction vectorin a second frame of reference to position the patient in the desiredpose with respect to the beam axis.
 11. A patient positioning system fora radiation therapy system having a plurality of components at leastsome of which are subject to uncontrolled movement, the positioningsystem comprising: a plurality of external measurement devices arrangedto obtain position measurements of the plurality of components withrespect to a substantially fixed frame of reference; a movable patientsupport configured to support a patient substantially fixed in positionwith respect to the patient support and to controllably position thepatient in multiple translational and rotational axes; and a controllerreceiving patient position information from the plurality of externalmeasurement devices and configured to provide translation and rotationmovement commands to the movable patient support to align the patient ina desired pose such that the positioning system compensates foruncontrolled movement of the plurality of components.
 12. Thepositioning system of claim 11, wherein aligning the patient in thedesired pose comprises aligning the patient support with respect to abeam delivery aiming point to which the radiation therapy system directsa radiation beam.
 13. The positioning system of claim 12, furthercomprising a patient imager system which obtains an image of the patientand a target iso-center, wherein the positioning system determines theposition of the target iso-center and aligns the target iso-center withthe beam delivery aiming point.
 14. The positioning system of claim 11,wherein at least one of the components subject to uncontrolled movementis also actively controllable and wherein the positioning systemprovides a feedback function to align the patient in the desired pose byproviding control signals to position the at least one activelycontrollable component.
 15. The positioning system of claim 11, furthercomprising a plurality of markers fixed to selected points of theplurality of the components subject to uncontrolled movement and whereinthe external measurement devices obtain multiple position measurementsof the markers so as to provide a more accurate measurement of a givenmarker's location than provided by a single measurement.
 16. A method ofregistering and positioning a patient for delivery of therapy with asystem having a plurality of components, wherein at least certain of thecomponents, including a patient positioner and a therapy nozzle, aresubject to uncontrolled movement, the method comprising the steps of:positioning a patient in an initial treatment pose with the patientpositioner; externally measuring the location of selected monuments ofthe plurality of components with respect to a substantially fixed frameof reference so as to define the initial treatment pose in thesubstantially fixed frame of reference; determining a difference vectorbetween the measured initial patient pose and a desired patient pose;and providing movement commands to the patient positioner to bring thepatient to the desired patient pose.
 17. The method of claim 16, whereinexternally measuring the location of the selected points of theplurality of components comprises employing a plurality of cameras toobtain optical images of the plurality of components.
 18. The method ofclaim 16, wherein providing the movement commands comprises transformingthe difference vector in the substantially fixed frame of reference intoa corresponding movement vector for the patient positioner in a secondframe of reference.
 19. The method of claim 16, further comprising thestep of obtaining a radiographic image of the patient and determiningthe difference vector based at least partially on the patient image. 20.The method of claim 19, wherein obtaining the radiographic imagecomprises the steps of: positioning an x-ray source and an x-ray imagergenerally along a therapy beam axis with the patient interposedtherebetween; exposing the patient and the x-ray imager via the x-raysource; and removing the x-ray source and imager from the therapy beamaxis.